1. Field of the Disclosure
The present disclosure relates to a focus detection sensor, an image pickup system, and a method for driving the focus detection sensor.
2. Description of the Related Art
In general, an image pickup apparatus having a function of autofocus (hereinafter referred to as “AF”) in which a focal distance of an image pickup lens is controlled in accordance with a state of detection of focus of an object detected by a focus detection sensor including photoelectric conversion elements so that the object is automatically focused has been widely used. Furthermore, control of charge accumulation periods and gains to be output of photoelectric conversion elements included in a focus detection sensor in accordance with brightness and contrast of an object is widely performed.
For example, Japanese Patent Laid-Open No. 2006-251777 discloses a focus detection sensor using line sensors including a plurality of photoelectric conversion elements (pixels). In the focus detection sensor, the line sensors are divided into a plurality of regions and accumulation is stopped when a difference (contrast) between a largest value and a smallest value of pixel signals exceeds a target value for each region. Furthermore, Japanese Patent Laid-Open No. 10-333021 discloses arrangement of a monitor sensor in the vicinity of a pair of line sensors used for focus detection and control of an accumulation (integration) time of the pair of line sensors performed in accordance with a signal supplied from the monitor sensor.
However, in the configuration disclosed in Japanese Patent Laid-Open No. 2006-251777, charges obtained by photoelectric conversion in the pixels are constantly transferred to an accumulation circuit, and therefore, noise generated in the accumulation circuit during accumulation is also accumulated with the charges obtained by the photoelectric conversion. When the accumulation period is long, an amount of the generated noise is increased, and therefore, adverse effect of a noise component on a result of the accumulation is not negligible and an error may occur as a result of focus detection.
Meanwhile, in the configuration disclosed in Japanese Patent Laid-Open No. 10-333021, since the accumulation control is performed in accordance with outputs of the monitor sensor disposed separately from the line sensors, charges obtained by photoelectric conversion performed in pixels during accumulation are not transferred to a memory circuit and a monitor circuit. Accordingly, noise to be generated may be reduced by resetting the memory circuit and the monitor circuit until the accumulation in the pixels is terminated. However, in Japanese Patent Laid-Open No. 10-333021, since the monitor sensor is disposed in the vicinity of the line sensors, when a plurality of line sensors are to be disposed, the monitor sensor restricts layout of the line sensors and arrangement of the monitor sensor is also restricted to perform light metering in high accuracy. As a result, an area of a chip of the focus detection sensor is increased, and therefore, cost is increased and miniaturization of an optical device including the focus detection sensor may not be attained.